1. Field of the Invention
The present invention relates to a method for manufacturing semiconductor devices and, more particularly, to a method of forming an alignment key on a semiconductor wafer.
2. Description of the Related Art
Semiconductor integrated circuit devices are made by forming a plurality of features in layers on a semiconductor wafer. The wafer is typically made from silicon or gallium arsenide, and photolithographic techniques are commonly used to form the features on the wafer. Each feature, when formed, must be aligned with the previously formed features in order for the integrated circuit to be properly constructed. To achieve this alignment, the usual practice is to provide alignment keys on the semiconductor wafer and corresponding alignment keys on each of the photolithographic masks used to form the features on the wafer. When the keys on the mask are aligned with the keys on the wafer, the features to be formed by the particular mask will be aligned with the features previously formed on the wafer.
FIG. 1 shows a semiconductor wafer 1 with integrated circuits formed on the wafer. The wafer 1 contains numerous die (or chip) areas 2 in which the integrated circuits are formed. The die areas 2 formed on the wafer 1 are separated from each other by vertical and horizontal scribe lines 3. As shown in FIG. 1, the horizontal scribe lines are evenly spaced and parallel to each other, the vertical scribe lines are evenly spaced and parallel to each other, and the horizontal and vertical scribe lines intersect at substantially right angles. After the wafer 1 has been processed to form the integrated circuits on each die area 2, the wafer 1 is cut into individual integrated circuit die by cutting along the scribe lines 3.
The conventional method of forming alignment keys on a semiconductor wafer will now be described with reference to FIG. 2. The alignment keys are formed on the scribe line areas at selected times during the fabrication process of the integrated circuit devices. As the fabrication process proceeds, the layer of material 13 formed on the surface of the semiconductor wafer substrate 10 increases in thickness. An alignment key 11 is formed by etching the layer 13 to produce channels in layer 13. After formation of the alignment key 11, a photoresist layer 12 is deposited over the entire surface of the semiconductor substrate 10. However, because of the thickness of the layer 13 and the resulting large depth of the channels forming the alignment key 11, the deposited photoresist layer 12 is often irregular and provides poor coverage of the channels, as shown in FIG. 2. This poor photoresist coverage of the alignment key 11 influences the die areas adjacent to the scribe line and may cause an undesireable "speed-boat" phenomenon.
This problem becomes more severe as more integrated circuit dies are packed more densely onto the wafer and the scribe lines are made narrower so that they occupy less area. This reduced width of the scribe lines also reduces the width of the channels forming the alignment keys and exacerbates the problem of poor photoresist coverage.